1. Field of the Invention
The present invention relates to a structure of a heat-spread substrate. More specifically, the present invention relates to a structure of heat-spread substrate in which a mounting ground pad is formed directly by a heat spreader.
2. Description of the Related Art
With the great improvement of the integrated circuit, the current semiconductor components have important features, such as multi-function, high integrity and high speed. Accordingly, for the sequent stage of package, many demand such as high density, high heat-spread performance and high reliability would be revealed. They are the important guides in development of the semiconductor package.
Referring to FIG. 1, a schematically cross sectional view of a conventional ball grid array type package with cavity down is shown. The structure of the ball grid array type package 100 with cavity down shown in the drawing is disclosed in U.S. Pat. No. 6,020,637. In the structure, the carrier is mainly consisted of a heat spreader 102, a ground plane 104, and a flex substrate 110. The heat spreader 102, the ground plane 104 and the flex substrate 110 are attached with insulating adhesives 106 and 108, respectively. The ground plane 104 and the flex substrate 110 form a cavity at the center. The flex substrate 110 is a usual flex tape interconnect substrate formed by laminating a patterned trace layer and a polyimnide layer. The patterned trace layer 112 on the top surface of the substrate has a mounting pad (or called as gold finger) and a ball pad. Moreover, the patterned trace layer 112 is covered by a solder mask 114. The die 120 has an active surface 120a and a back surface 120b. The die 120 is attached to the bottom of the cavity by applying an adhesive 124 on the back surface 120b. The ball pad 122 of the active surface 120a is connected to the ground ring 116 of the ground plane 104 and the mounting pad of the rigid substrate, respectively. The die 120, the gold wires 126, the mounting pad are encapsulated with a molding compound 128. Solder balls 130 are mounted on the surface of the ball pad 112. In the conventional ball grid array type package 100 with cavity down, the ground plane 104 is connected to the patterned trace layer 112 on the surface of the flex substrate 110 by a via 118. Then, the ground plane 104 is interconnected through solder balls 130. The via 118 is obtained by fill hole process. This would make the assembling complicated and adversely affect the bondability of the via 118 to the ground plane 104 to reduce the reliability of the product.
It is one object of the present invention to provide a heat spreader, in which a ground pad is directly provided on the heat spreader and solder balls are mounted so as to ensure the reliability of the joints where the heat spreader is grounded.
It is another object of the present invention to provide a heat spreader, in which a ground pad and a substrate-supporting surface are simultaneously formed by thickness decreasing process to simplify the fabricating process. Further, it also can ensure the coplanarity of solder balls to enhance the reliability of the product by thickness controlling to make the ground pad and the position of the solder balls locate in the substrate stand at the same level.
To achieve the above and other purposes, a heat-spread substrate consisting of a metal heat spreader and a substrate is provided. The metal heat spreader has a surface with a cavity, which is adapted for supporting a die. Such surface further includes a ground ring arranged at the periphery of the cavity; a substrate-supporting surface surrounding the periphery of the ground ring; a plurality of first ground pads arranged at the periphery of the substrate-supporting surface; and a plurality of second ground pads arranged on the substrate-supporting surface and protruding it. The substrate is provided on the substrate-supporting surface having a plurality of through holes. The through holes corresponds to the first ground pad so as to make it be located therein, respectively. The substrate further includes a plurality of mounting pads and ball pads, in which the mounting pads are close to the cavity. The first ground pads, the second ground pads and the ball pads are formed in the form of ball grid array and are coplanar roughly.
To achieve the above and other purposes according to the present invention, a fabricating process of the above heat-spread substrate is provided. The process comprises at least the steps as following. First a metal heat spreader is provided. A thickness decreasing process is carried out on one of the surfaces of the metal heat spreader to decrease the thickness of part of areas on the surface. A cavity, a ground ring, a substrate supporting surface, first ground pads and second ground pads mentioned above are formed on the surface. In this case, the cavity, the ground ring, the substrate-supporting surface, the first ground pads and the second ground pads have different decreasing thickness, respectively. A plating process is then carried out to form a plating layer on the surfaces of the first ground pads and the second ground pads, respectively. The substrate is attached on the substrate-supporting surface. As set forth above, the substrate has a plurality of through holes to make the first ground pads locate in the corresponding through holes, respectively. The substrate further includes a plurality of mounting pads and a plurality of ball pads. In the substrate, the mounting pads are close to the cavity. The first ground pads and the second ground pads and the ball pad are formed in the form of ball grid array and are coplanar roughly.
According to one of the preferred examples of the present invention, when the above heat-spread substrate is applied in package, the die is attached in the bottom of the cavity by the back surface thereof. The ball pads on the active surface of the die are connected to the ground ring and the mounting pads of the substrate by leads. The whole cavity and the parts connected to leads are encapsulated with the molding compound. Solder balls are mounted in the first ground pads, the second ground pads and the ball pads to form a structure of a ball grid array type package. In the present invention, the substrate can be a rigid substrate or a flex substrate. A plating layer can be formed on the surfaces of the first ground pads, the second ground pads and the ball pads to improve the bondability of the solder ball to them.
In the process according to the present invention, a black oxidation can be further carried out after plating process to oxidize the substrate-supporting surface to improve the bondability of the substrate to the metal heat spreader. The thickness decreasing process includes stamping. Alternatively, it can be accomplished by multi-photography.